CONCRETE IN HOT ENVIRONMENTS - CHAPTER 6 pdf

Typicalvertical formwork systems utilized in construction include conven-tional formwork, ganged forms, jump forms, slipforms, and self-raising forms.. 6.2 CONVENTIONAL WALL/COLUMNS FORM

6.4 Jump Forms

Vertical formwork systems are those used to form the verticalsupporting elements of the structure—columns, core walls, andshear walls The functions of the vertical supporting systems are

to transfer the floor loads to the foundation and to resist the lateralwind and earthquake loads Consequently, the construction of ver-tical structural elements precedes flat horizontal work Typicalvertical formwork systems utilized in construction include conven-tional formwork, ganged forms, jump forms, slipforms, and self-raising forms

Formwork systems for vertical concrete work can be fied into two main categories, namely, crane-dependent systemsand crane-independent systems Gang formwork and jump form

classi-are classified under crane-dependent systems On the other hand,slipform and self-raising formwork are classified as crane indepen-dent systems in which formwork panels are moved vertically byother vertical transportation mechanisms This chapter focusesprimarily on crane-dependent formwork systems and their applica-tion and limitations.

The conventional wall system is the only hand-set system.The other four formwork systems are made of prefabricated modu-lar panels before they can be transported by cranes or any othervertical transportation system

6.2 CONVENTIONAL WALL/COLUMNS FORMING SYSTEMS

This all-wood forming system consists of sheathing made of wood or lumber that retains concrete until it hardens or reachesadequate strength This system is also known as job-built woodsystem The sheathing is supported by vertical wood studs Thestuds are supported by horizontal wales which also align theforms Single or double horizontal wales are used to support thestuds (Figure 6.1) However, double wales are preferred to avoiddrilling through single wales, which reduces its load-carrying ca-pacity Ties are drilled through wales (single wale) or inserted be-tween them (double wale) to resist the lateral pressure of plastic(wet) concrete An inclined bracing system is used to resist con-struction and wind loading that formwork is subject to

ply-6.2.1 System Components and Construction

Sequence

Components of the conventional wall system are similar to tional wood system components for slabs but have different names.Joists become studs and stringers become wales Also, the twosystems are similar in that they are built in situ and stripped piece

conven-by piece

Fig 6.1 All-wood conventional wall-forming systems.

Erection sequence for all wood conventional wood system is

as follows:

1 Erection of wall form starts by attaching the first side ofthe plywood to the concrete footing wood sill (shoe) byanchors or hardened nails The plywood is erected withthe longer direction parallel to the length of the wall

2 Studs are then erected and temporarily supported bywood bracing[usually 1⫻ 6 in (25.4 ⫻ 152.4 mm) brace].Reinforcing steel, opening boxouts, and other electrical

or mechanical systems are installed before the secondside of the wall is erected The plywood is then nailed

to the studs and the other wall side (plywood) is thenerected

3 Tie holes are then drilled from both sides of the wall atproper locations.

4 Wales are then erected and attached to the outside of thestuds by nails In double-wale systems, each wale should

be located above and below the tie location

5 Bracing is then installed to support horizontal loads sulting from wind loads and concrete vibration

re-6 To facilitate concrete placement and finishing, scaffoldsare erected and attached to the top of the wall

6.2.2 Formwork for Columns

Conventional formwork for columns is made of sheathings nailedtogether to form rigid sides Typically, formwork for concrete col-umns has four sides Column form sides are held together byyokes or clamps (see Figure 6.2) Another function of these yokes

is to prevent the buckling of sheathing resulting from the tal lateral pressure when the fresh concrete is placed

horizon-Concrete lateral pressure is greater near the bottom of theform As a result, yokes are spaced at smaller intervals near thebottom than near the top of the form Column form sides may also

be tied by straps or steel angles In order to prevent breaking ofthe corners or edges, it is common practice to add a triangularfillet to the form along the edges of the columns This practicealso facilitates stripping of column forms

Columns can take several shapes: round, rectangular, shaped, or various irregular shaped cross sections Irregularshapes are frequently formed by attaching special inserts insidesquare or rectangular forms

L-6.2.3 Erection of Column Forms

Erection starts by marking a template on the floor slab or footing toaccurately locate the column floor Erection sequence is somewhatsimilar to wall forms; however, methods vary depending on theavailable lifting equipment and whether reinforcing cages andforms are built in place or not

Figure 6.2 Formwork for columns.

6.2.4 Tie Rods

The functions of tie rods are to resist the tensile forces resultingfrom the pressure of fresh concrete and to hold the two sides ofwall form (sheathing) at the correct thickness Wood or metalspreaders can also be used to keep the thickness of the wall con-stant Ties can be broken off or unscrewed and remain an integralpart of the concrete wall Other types of ties may be removed forreuse, resulting in visible holes Holes can be left visible or filledwith mortar or ready-made plugs Figure 6.3 shows several types

of tie rods used in forming concrete columns and walls carrying capacity for ties ranges from 1,000 to 70,000 lb (450 to31,750 kg)

Load-Figure 6.3 Wall form ties.

6.2.5 Construction Practices

1 It is good practice to minimize cutting formwork material

to suit the wall size Plywood, studs, and wales may beextended beyond the size of the wall and concreting isstopped at the appropriate size For example, the draw-ings may call for a wall to be 11 ft (3.35 m) high Plywoodand studs can be extended to 12 ft (3.66 m) high andconcreting can be stopped when it reaches 11 ft (3.35 m)high

2 In long studs or wales where more than one piece isneeded, joists between different pieces should be stag-gered to avoid creating a plane of weakness

3 When placing concrete for tall columns, it is mended to have pockets or windows at mid-height orother intervals to facilitate placing and vibrating the con-crete

recom-6.2.6 Economy of Conventional Wall Formwork

Conventional wall formwork systems are economical when a ited number of reuses are expected and wall or column configura-tions are not repetitive The expected number of reuses for conven-tional job-built forms is three to four times, depending on thequality of wood, connecting hardware, and handling of the woodduring erection and stripping

lim-The limitations of using conventional wood systems for crete walls are similar to those of conventional slab forms, namely,high labor costs and materials waste

con-6.3 GANGED FORMING SYSTEMS

Ganged forms are large wall form units that are made of panelsjoined together with special hardware and braced with strong-backs or special steel or aluminum frames Gang forms can bemade on the site, rented, or purchased from formwork manufactur-

ers The advantages of manufactured forms over site made is thatthey are precise in dimension and can be reused a larger number

of times

6.3.1 Sizes and Materials

Sizes of gang forms vary substantially from smaller units that arehandled manually, to much larger units that are handled and raised

by cranes Smaller gang forms are typically 2 ⫻ 8 ft (0.61 ⫻ 2.44m) and 4 ⫻ 8 ft (1.22 ⫻ 2.44 m), and weigh between 50 and 100

lb (23 and 45 kg) Larger gang forms are limited by crane carryingcapacity and can reach 30⫻ 50 ft (9.1 ⫻ 15.2 m) Some literaturerefers to smaller gang units as ‘‘modular forms,’’ and to the largerunits as ‘‘gang forms.’’

Gang forms can be made of aluminum (all-aluminum), wood face and aluminum frame, plywood face and steel frame, andsteel All-aluminum gang forms consist of aluminum sheathingsupported by an aluminum frame along with intermediate stiffen-ers The aluminum sheathing can be plain or take the shape of abrick pattern for architectural finish Aluminum sheathing is notpopular because of its relatively higher cost and the tendency ofconcrete to react chemically with aluminum A common modulefor all-aluminum gang is 3⫻ 8 ft (0.91 ⫻ 2.44 m) panels

ply-A more popular and widely used alternative to the num gang forms is the aluminum frame with plywood This system

all-alumi-is lighter and less expensive than the all-aluminum gang form wood is attached to the aluminum frame by aluminum rivets.Another method of attaching plywood to aluminum beams is

Ply-to use the nailer-type joists in the assembly of the gang form Theplywood is nailed to the nailer type beam by regular nails A com-mon module for this system is 2 ⫻ 8 ft (0.61 ⫻ 2.44 m) panels.Figure 6.4 shows a gang form with aluminum frame and plywoodface

The third type of gang forms consists of a plywood face ported by steel walers Walers are typically made from doublechannels to allow ties to be inserted between the channels and toreduce the deflection of the gang form The advantage of this sys-

sup-Figure 6.4 Aluminum frame gang form.

tem over the above mentioned systems is its ability to carry greaterloads at longer distances between walers A common module forthis system is the 4 ⫻ 8 ft (1.22 ⫻ 2.44 m) panel

The all-steel gang form is made of steel sheathing and steelstuds and wales This system is used to support fresh concrete forhigh, thick, and multiple lifts This system has an unlimited num-ber of reuses as long as good storage practices are followed Acommon module for this system is the 2 ⫻ 8 ft (0.61 ⫻ 2.44 m)panel because of its heavy weight

6.3.2 Gang Forms Assembly

Ganged forms are assembled on the ground, raised into place, andstripped as one unit Assembly of gang forms starts by placing thewalers above lumber blocks on flat and level ground For fasterand more efficient assembly, a gang assembly table can be usedinstead of assembling the gang on the ground Walers are thenleveled, aligned, and locked in their proper position The nailer-

type beams are then placed on, and perpendicular to, the walers.The nailer-type beams are attached to the walers by clips Twolumber-end pieces are then placed and attached to the walers Theplywood is then placed and fastened by screws Tie rod holes can

be placed on the ground; however, it is good practice to drill holesand insert tie rods when gang forms are erected to ensure thatholes on the two sides of plywood are matched

6.3.3 Economy and Advantages of Gang Formwork

1 Productivity of gang forms is higher than traditionalforms because they are assembled on the ground andstripped as one unit

2 Gang forms produce high-quality smooth concrete withfewer joints Also, form liners can be attached on the ply-wood to produce architectural concrete

3 Gang forms have higher reuse value than traditional wood formwork systems Also, plywood can be replacedwithout any need to replace the supporting frame

all-6.3.4 Limitations of Gang Formwork

1 The major limitation of gang formwork is that beforemoving gang forms vertically or horizontally to the nextpouring position, they have to be brought down to theground for cleaning and oiling This process substantiallyincreases the cycle time between two lifts

2 Gang forms are not suitable for small walls or walls rupted by pilasters or counterforts

inter-3 Because of their large sizes, safety is a major concernwhen moving ganged forms

6.4 JUMP FORMS

Jump form systems are used where no floor is available on which

to support the wall formwork, or the wall and column proceedahead of the floor Jump forms consist of a framed panel attached

to two or more strongbacks They can be one-floor high, supported

on inserts set in the lift below, or two sets can also be used, eachone-floor high that alternately jump past each other (Figure 6.5)

6.4.1 Jump Form Components

Jump forms consists of two parts: an upper framed panel form withits supporting system and working platform, and a supportingstructure that is attached to the concrete wall below the wall beingplaced The function of the upper framed panel form is to supportthe freshly placed concrete The supporting structure is attached

to a stiff concrete wall Its function is to support the upper framedpanel form Jump form components are shown in Figure 6.6

Upper Framed Panel Form

The upper part consists of three main elements: (1) framed panelform, (2) supporting brace, and (3) working platforms The framedpanel form consists of a plywood face supported by two or morestrongbacks The frame panel form and the stongbacks are sup-ported by an adjustable pipe brace The brace is used for plumbingand stripping of the frame panel form The strongback beams andthe pipe brace are rested and connected to a horizontal beam that

is anchored to the top of the concrete wall underneath the wallbeing poured The strongbacks, brace, and the horizontal beamare forming a truss system that supports the freshly placed con-crete Another function of the horizontal beam is to support thewalkway under the lower working platform

After the concrete gains enough strength to support its ownweight, the framed panel form is moved away from the concretewall to allow the attachment of landing brackets for the next pour-ing position and to finish concrete patching The framed panelform is moved away by either tilting or moving horizontally byrollers away from the concrete wall

There are two working platforms in the upper framed panelform The upper working platform is used to place and vibrate con-

Figure 6.5 Jump form.

Figure 6.6 Jump form components (Courtesy of SYMONS Corp.)

crete and to attach the landing bracket ( jump shoe) The lowerworking platform has two functions: (1) to allow constructionworkers to remove form ties and anchor bolts and (2) to clean andre-oil form panels.

The Supporting Structure

The supporting structure is basically a support mechanism for theframed panel form and its working platforms The lower part ofthe supporting structure is used as a walkway for repair work ofconcrete A ladder can be used and extended between the walkwayand the horizontal beam for repair work along the wall height

6.4.2 Typical Work Cycle

First Lift

Figure 6.7 shows a typical first lift on grade, using the jump formsystem in the same manner as gang formwork The first lift isformed by the framed panel form with its strongback and the com-pression brace In this case, the wall braces can be anchored di-rectly to the ground or slab for form alignment It should be notedthat a slab on grade or foundations should be available in order

to start jump forms from the ground level

Second Lift

After placing the concrete for the first lift, the tie rods are releasedand the form is then lifted to the next pouring position The secondlift begins by attaching the jump shoe to the wall at the first ‘‘jump’’elevation The framed panel form is attached to the crane slingsand hoisted into position above the jump shoes The lower support-ing structure is then attached without the lower overhanging walk-way (Figure 6.8)